The present invention relates to a variable power assist steering system for a vehicle.
Among known power steering mechanisms, U.S. Pat. No. 4,561,521 discloses a power steering mechanism for an automotive vehicle having a pressure operated vehicle steering mechanism controlled by a rotary valve with primary and secondary valve portions. The primary valve portion develops a fast-rising steering pressure characteristic for incremental changes in steering torque at low driving speeds and a flatter or more gradual steering pressure rise characteristic for incremental changes in steering torque at high driving speeds. The latter resulting from the action of the secondary valve portion which functions in cooperation with the primary valve portion whereby the pressure versus torque relationship is optimized for parking maneuvers, for driving maneuvers and for center-feel. The change from the low speed steering mode to the high speed steering mode being gradual whereby the center-feel increases in proportion to increasing road speed. A speed sensitive valve is used to control fluid flow from the outlet side of a pump to the secondary valve portion so that at high vehicle speeds a parallel flow path is provided between the steering control valve and the pump as fluid is distributed to both primary and secondary valve portions. At low vehicle speeds, a speed sensitive valve restricts the flow of fluid from the pump to the secondary valve portion. Above a predetermined speed the speed sensitive valve opens thereby allowing fluid to be distributed from the pump to secondary valve portion in parallel with respect to the flow to the primary valve portion. A change from a high degree of power assist to a low degree of power assist, and conversely is effected by a variable force solenoid which is used to establish a parallel flow path from the pump to the secondary valve portion through a variable flow orifice. A speed sensing module controls the solenoid to open and close a variable orifice valve thus providing gradual changes in the degree of power assist as the vehicle speed changes.
U.S Pat. No. 4,570,735 discloses a power steering gear mechanism having a pressure operated vehicle steering mechanism controlled by a rotary valve with primary and secondary valve portions. This rotary valve is similar to the above mentioned known rotary valve.
The known steering control valves disclosed by the above two U.S. patents use a rotary valve with primary and secondary valve portions which are arranged out of position in the axial direction along the axis of rotation of the rotary valve. The valve grooves for the rotary valve are difficult to manufacture and require skilled labor. The arrangement of the primary and secondary valve portions causes increased axial length of the rotary valve.
In order to solve the above problem, a new power steering system has been proposed as disclosed in co-pending U.S. patent application Ser. No. 044,065, filed August 29, 1987. In view of the co-pending application, a main feature of the present invention resides in a system for controlling the operation of a control valve in response to a vehicle control factor other than steering torque, as a vehicle speed.